RAAC differs from conventional concrete mainly in that it is filled with air bubbles instead of aggregates such as gravel. It is lighter, easier to build quickly, and less expensive than other forms of concrete. The air bubbles also provide good thermal insulation, meaning buildings containing RAAC are easier to heat and cool. It was widely used in post-war Britain until the 1990s for casting panels for roofs, floors and walls, and was particularly popular in the public sector, where it was used to rebuild schools, hospitals and other infrastructure.

But everything cheap and fast has a price. RAAC, being less durable than standard concrete, gradually weakens and bubbles allow water to seep through. While the steel bars that support RAAC panels are usually covered with waterproof layers, a lack of maintenance can cause these to corrode, further weakening the concrete. panels and causing them to break. The lifespan of a RAAC structure is only between 30 and 50 years. This vulnerability has been known for years. But over the past month it has taken on the magnitude of a present-day crisis, as it becomes clear that many important buildings and pieces of infrastructure are well past the end of their service life. As well as schools and hospitals, RAAC problems have been seen in theatres, apartment blocks, municipal buildings and even London’s two largest airports, Heathrow and Gatwick. This has created a multi-million dollar headache for the UK government and illustrates once again the cost of underinvesting in public goods and relying on quick fixes to meet long-term needs.

“The problem with these panels is not so much the material itself. It’s the fact that they have been used well beyond their expiry date,” says Juan Sagaseta, reader in structural robustness at the University of Surrey. “Unfortunately, in our society, spending on new buildings and opening new schools or hospitals is often seen as more glamorous than spending on maintaining old ones.”

Issues surrounding RAAC were first studied in the 1990s by the Building Research establishment (BRE), an organization initially established as a government agency that now operates as a social enterprise. At the time, there were concerns about the removal of roof panels from some buildings, although there was no conclusive evidence of immediate safety risks. It was not until 2018 that the Department for Education finally took action, after the ceiling at a primary school in Kent, southern England, suddenly collapsed. Fortunately, the incident happened on a Saturday and no one was injured. The school had been rebuilt in 1979 using the RAAC after a fire. School authorities were sent questionnaires trying to determine whether or not they had RAAC in their buildings, but, Sagaseta says, they (understandably) often did not have the expertise or resources to identify the material. Finally, in the fall of 2022, the Ministry of Education sent professional surveyors to classify RAAC construction as “critical” or “non-critical”.

The sudden decision to close schools this summer was triggered by three cases of RAAC panels which were considered non-critical but subsequently failed. The first incident involved a commercial building, the second a school in another country and the third an English school in late August. The approximately 150 establishments now known to be most at risk represent just a tiny fraction of England’s 22,000 public schools, colleges and nurseries.